Liquid natural gas (“LNG”) terminals continuously generate boil off gas (“BOG”). This BOG is generated primarily due to heat leak from the atmosphere through tank insulation, unloading and recirculation line insulation, and from pumping energy and gas displacement from LNG storage tanks during ship unloading.
It is relatively easy to condense BOG in a counter flow packed tower, that is, a packed bed type condenser. However, much more BOG is created during unloading operations than during normal send-out operations.
There can be difficulty in controlling pressure during transient stages of operations, such as beginning or ending ship unloading. For send-out operations to remain uninterrupted, it is important that BOG condenser pressure be maintained. Failure to do so can create interruptions in delivery, because LNG send out pumps may trip and interrupt send-out flow.
Conventional BOG condensers comprise a condenser stack (a packed section) on the top of the condenser and a surge drum at the bottom. The surge drum is intended to provide surge volume for the LNG send out pumps. LNG from the send out pumps is commonly divided into two flow streams. The first LNG flow stream enters the top of the packed bed and is flow controlled. This first LNG flow stream is used to condense BOG introduced into the condenser. The second LNG flow stream is either fed into the top of the bottom drum and is level controlled, or can be connected to the suction header of the LNG send out pumps.
In such conventional designs, a BOG unloading compressor feeds BOG into the condenser stack at the bottom of the packed bed, to be condensed in the packed bed by the first LNG flow stream. The bottom section must simultaneously serve as a surge vessel for the LNG send out pumps and maintain a liquid level. Thus, this design is sensitive to transitions in operating volume, such as a substantial increase in BOG volume during ship offloading. It is desirable to reduce such pressure fluctuations and thereby improve the reliability of the terminal during all phases of operation.
Accordingly, it is an object of the invention to provide a BOG condenser which can maintain an appropriate operating pressure in multiple operation phases.
It is another object of the invention to provide a BOG condenser which is pressure-controllable through transient phases of terminal operation.